This invention relates to an antenna device and, more particular, to an antenna device for use in a frequency band of a wireless Local Area Network (LAN).
In the manner which is well known in the art, the wireless LAN is an LAN using a transmission path except for a wired cable, such as electric waves, infrared rays, or the like.
Standardization of the wireless LAN is developed in IEEE (Institute of Electrical and Electronics Engineers) 802.11 Committee. That is, the IEEE 802.11 Committee develops specifications of the standard of the wireless LAN.
For example, IEEE 802.11a is a specification of a high-speed wireless LAN and a wireless access for 5 GHz band where the IEEE 802.11 Committee develops. A communication rate (a transfer rate) is about 20 Mbits/sec to 50 M bits/sec. A CSMA/CD (carrier sense multiple access with collision detection) is used as an MAC (media access control). A modulation method of a physical layer is an OFDM (orthogonal frequency division multiplex).
On the other hand, IEEE 802.11b is a specification of the wireless LAN where the IEEE 802.11 Committee standardizes in September, 1999. The IEEE 802.11b uses frequencies of 2.4 GHz band and uses a direct spread (DS) as a modulation method. A transmission rate (a transfer rate) is 11 Mbits/sec or 5.5 Mbits/sec.
Furthermore, IEEE 802.11g is one of standards for the wireless LAN where the IEEE 802.11 Committee develops in June, 2003 and a specification for carrying out communications about 54 Mbits/sec at 2.4 GHz band. The OFDM is used as a modulation method. Accordingly, the IEEE 802.11g uses the frequencies of 2.4 GHz band which is similar to that of the IEEE 802.11b and supports the transfer rate of 54 Megabits/sec which is about five times of that of the IEEE 802.11b. In contrast to the IEEE 802.11a for supporting the transfer rate of 54 Mbits/sec, the IEEE 802.11g maintains compatibility with the IEEE 802.11b. In addition, although a maximum transfer rate of 54 Mbits/sec is similar to that of the IEEE 802.11a. the 2.4 GHz band is a “busy” frequency band where a lot of equipments except for the wireless LAN use. Therefore, it is said that a real transfer rate in the IEEE 802.11g becomes later than that of the IEEE 802.11a.
Inasmuch as the IEEE 802.11b and the IEEE 802.11g use the same use frequency band of 2.4 GHz band in the manner which is described above, both are collectively called IEEE 802.11b/g herein.
Various antenna devices used in the frequency band of the wireless LAN are already known in the art.
By way of example, JP 2003-152429 A (which will later be called Patent Document 1 and which corresponds to U.S. Pat. No. 6,917,333 B2) discloses a flat-plate antenna device capable of stably exhibiting desired antenna characteristics. The flat-plate antenna disclosed in Patent Document 1 comprises a conductive flat plate and a power supply line (a feeding line). The conductive flat plate has a slit portion with a width proportional to a frequency band width and comprises a radiating element portion disposed on one side of the slit portion and a ground portion disposed on the other side of the slit portion. The power supply line (the feeding line) has a first conductor directly connected to the radiating element portion and a second conductor directly connected to the ground portion. Length of the radiating element portion contributes to resonance frequency, width of the slit portion contributes to frequency band, and ratio between length of the conductive flat plate and width of the ground portion contributes to directivity.
In addition, JP 4,780,352 B (which will later be called Patent Document 2) discloses an inexpensive antenna device (a sheet plate antenna) which is capable of easily assembling and of improving mounting strength for a coaxial cable. The antenna device disclosed in Patent Document 2 comprises an antenna device which is capable of transmitting and receiving a radio wave having a desired frequency band of 2.4 GHz band and which comprises the coaxial cable having a center conductor, an external conductor, and a sheath covering the external conductor, and an antenna element. The antenna element is made of a metallic plate which comprises an antenna pattern portion configured with an inverted-F antenna and a ground portion formed integrally with the antenna pattern portion. The metallic plate is, for example, formed from phosphor bronze. The coaxial cable is swaged and fixed to the ground portion and the center conductor of the coaxial cable is connected to a feeding portion of the inverted-F antenna.
Furthermore, JP 2011-19178 A (which will later be called Patent Document 3 and which corresponds to US Publication 2012/0105303 A1) discloses an antenna device (a board antenna) which is capable of easily soldering an external conductor of a coaxial cable to a ground pattern portion. The antenna device disclosed in Patent Document 3 comprises an antenna device which is capable of transmitting and receiving a radio wave having a desired frequency band of 2.4 GHz band and which comprises the coaxial cable having a center conductor and an external conductor and an antenna element. The antenna element comprises an antenna pattern portion and a ground pattern portion. The center conductor of the coaxial cable is electrically connected to a first solder portion of the antenna pattern portion by soldering while the external conductor of the coaxial cable is electrically connected to a second solder portion of the ground pattern portion by soldering. The ground pattern portion has, in vicinity of the second solder portion, a ground pattern opening portion defining the second solder portion. The second solder portion is sandwiched between the first solder portion and the ground pattern opening portion.
However, inasmuch as each of the antenna devices disclosed in the above-mentioned Patent Documents 1-3 is configured so that the radiating element portion (the antenna pattern portion) comprises the inverted-F antenna, they are disadvantageous in that a frequency band of a transmittable/receivable radio wave (radio signal) is narrow and radiation efficiency is also not excellent. In a case where the frequency band is narrow, on producing the antenna devices in quantity, problem arises when a frequency drift occurs. As a result, it reduces yields of quantity production.